Foils of the type described above are known. They are based generally on polyvinyl chloride. Polyvinyl chloride has the advantage of good HF-weldability and also of the transparency which is required in individual cases. PVC-products have been used for technical applications for many years and in the most diverse products. Owing to the trend of reducing the application of materials containing chlorine and plasticisers in all areas of life, there exists in the construction, automobile and cable sectors an urgent requirement for halogen-free alternative products.
A problem in the search for halogen-free alternative products often comprises the fact that PVC foils can very readily be welded by high frequencies as against which possible PVC replacement materials, such as for instance polyurethane foil or known thermoplastic polyolefin foils cannot be welded, or only with difficulty.
A further characteristic, which plays a role especially in the automobile sector, is sufficient cold flexibility of the material. High cold flexibility is especially important if the materials are subjected to dynamic loading at low temperatures. Thus for instance polyolefin foil materials serve as decor for covered airbag systems. In order for such a material to be, suitable for such an application it must be ensured that it does not become brittle or have a high frictional index at the test temperatures (customarily −30° C.), whereby the penetration of the décor by the airbag flap would be impaired. It must also be provided that the material does not splinter because of its embrittlement. These splinters could lead to injuries to the passengers. Consequently embrittlement of the foil material must be avoided and a desired opening of the airbag at low temperatures (free from flying particles) must be ensured.
High cold flexibility is also for instance important in storage space covers, which can be realised in the form of sheets or rollers where even in cold conditions satisfactory on and off rolling must be assured. At the same time for applications in automobile or vehicle areas stability under heat of at least 100° C. must be provided. Heat stability in this case means that the surface structure or finish must remain in the original condition following a 21-day storage at this test temperature and furthermore no additional sheen is to be seen.
The PVC-products mentioned fulfill a major part of the present requirements being discussed. They are especially also transparent, which is required in individual cases. Nevertheless they exhibit the disadvantage that they include plasticisers with their known disadvantages. Here especially the insufficient ageing stability and the emissions caused by the plasticisers should be mentioned. Both in automobile and also in the shoe sector there are strong tendencies to replace the widespread PVC materials by halogen-free substances. However many characteristics of polyvinyl chloride can only be displayed by other material classes after very high innovative expenditure.
There are indeed certain proposed solutions, which however are not altogether fully satisfactory. Here we should mention EP 0 703 271 A1. This describes a polyolefin composition, which is thermoplastic, halogen-free and flexible at room temperature. Here ethylene vinyl acetate-copolymers as well as polyethylene with very low density are used. Although these formulations are often useful for several applications they have the disadvantage that they do not exhibit sufficient heat stability as soon as they are exposed to temperatures greater than 80° C.
The state of the technology according to DE 100 18 196 A1, describes the method of production of a surface finished foil from a mass with a content of non-interlaced polyolefins and possibly other additives, whereby the foils obtained in the normal manner by obtaining a surface finish stability suitable for deep drawing are processed using electron beams, and the surface finished foil is deep drawn to a density of about 0.7 to 1.2 g/cm3. As non-interlaced polyolefins polypropylene, polyethylene, polypropylene-copolymers or terpolymers with C2, C4 to C12-α-Olefins and/or polyethylene-copolymers or terpolymers with C3 to C12-α-Olefins are used. The foils obtained are distinguished especially by good surface finish stability and can be stamped and laminated. They are however not HF weldable.
DE 196 53 590 C2, similarly concerns a PVC-free and thereby also halogen-free high frequency weldable polymer mixture. It must include a thermoplastic polyolefin comprising a polyamide as well as other copolymers, ethylene for instance but is however not transparent, owing to the thermoplastic polyolefin which contains polyamide. In accordance with the post-published DE 101 53 408, a high frequency weldable polymer mixture is similarly described which contains some 1 to 30% by weight of an ethylene copolymer with a content of polar comonomers below about 25% by weight selected from the group of vinylester of saturated C2–C8-carboxylic acid and C1–C12-alkylacrylate and/or methacrylate, about 1 to 40% by weight of a terpolymer or a further copolymer with a content of comonomers above about 25% by weight, and about 35 to 75% by weight of a thermoplastic polyolefin with a melting point above about 115° C. This polymer mixture is however not transparent owing to the last named component.
It is consequently the aim of the current invention to propose foils or molded bodies which do not have a basis in PVC and consequently are free from halogens, however continue to show the advantageous characteristics of PVC foils, namely transparency, HF weldability and stability under heat, a desirably good flexibility when cold and stability under ageing and furthermore also exclude or minimise the disadvantageous emissions which are associated with the application of PVC foils.